Composite current-limiting fuse structures



Jan. 27, 1959 I K. w. SWAIN ETAL 2,

COMPOSITE CURRENT-LIMITING FusE STRUCTURES Filed May 2, 1957 3 Sheets-Sheet 1 K. w. SWAIN ET AL 2,871,314

' COMPOSITE CURRENT-LIMITING FUSE STRUCTURES Jay. 27, 1959 3 Sheets-Sheet 2 f 1 I: p,

Filed May 2. 1957 Jan. 27, 1959 K. w. SWAIN ET AL 2,

COMPOSITE CURRENT-LIMITING FUSE STRUCTURES Filed May 2, 1957 s Sheets-Sheet 3 [220222212215.- Kenafia/ W/Swain, F' edeaz'nfi Jfiiazaciia,

Ek wfiz Jaizefl,

49 PM Ma -W COMPOSITE CURRENT-LIMITING FUSE STRUCTURES Kenneth W. Swain, Hampton Falls, and Frederick J. Kozacka, South Hampton, N. H., and Erwin Salzer, Waban, Mass, assig'nors to The Chase-Shawmut Company, Newhuryport, Mass.

Application May 2, 1957, Serial No. 656,661

Claims. ('Cl. 200-120) This invention refers to current-limiting fusible devices, i. e. to fusible devices which operate sufficiently fast on the occurrence of relatively large fault currents to effect interruption of the faulted circuit before the fault current reaches the available peak value. I

It is one object of the invention to provide currentlimiting fusible devices which operate very rapidly in the presence of relatively onerous interrupting conditions such as, for instance, a relatively high circuit voltage.

Another object of the invention is to provide currentlirniting multibreak devices having ribbon-type fuse link means comprising a plurality of serially related points of reduced cross-sectional area sufficiently narrow and sufliciently short to approximate point heat sources when said fuse link means are carrying current, which fusible devices have operating characteristics similar to thoseof current-limiting fuses having ribbon-type fuse link means defining one single point-heat-source.

Another object of the invention is to provide currentlimiting fusible protective devices having general operating characteristics similar to those of the current-limiting fuses disclosed in United States Patent 2,734,111 to Frederick J. Kozacka, Low-Voltage High Capacity Current- Limiting Fuses, February 7, 1956, but being adapted to be applicable in the presence of considerably more onerous interrupting conditions than the aforementioned prior art current-limiting fuses.

The fusible conductor of a current-limiting fuse may be either in the shape of a wire or of a ribbon. Ribbontype fuse links are indicated wherever the current-carrying requirements are relatively high, and the present in vention refers to current-limiting fuses or current-limiting fusible devices having ribbon-type fuse link means. V

It is common practice to provide ribbon-type fuse link means with one or more points of reduced cross-sectional area or necks. One object of such a neck or necks is to fixedly establish the point or points of initial fusion and arc-formation along the fuse link. The geometrical configuration of a neck or point of restricted cross-sectional area determines the speed of response of a currentlimiting fuse. Where it is imperative to combine highest possible speed of response with highest possible currentcarr'ying capacity the neck or point of reduced cross-sectional area of a ribbon-type fuse link must be sufficiently narrow and sufficiently short to approximate a point heat source when the link is carrying current.

Heretofore current-limiting fuses having ribbon-type fuse links with one single neck or portion of reduced cross-sectional area sufficiently narrow and sufficiently short to approximate a point heat source when carrying current could only be applied up to a certain relatively low interrupting rating. In the presence of interrupting conditions exceeding these ratings current-limiting fuseswith links forming a single point-heat-source-neck will generally not operate properly, i. e. the fuses tend to restIike after initial interruption of the faulted circuit. As a general rule the limitations inherent in single neck fuse atent "ice links can be avoided by providing a plurality of necks, or points of reduced crosssectional area, along a fuse link. This gives rise to increased arc voltages. In the case of current-limiting fuses with ribbon-type fuse links defining a single point-heat-source-neck the number of necks cannot readily 'be increased to increase the arc voltage to cope with more onerous interrupting conditions. increasing the number of point-heat-source-necks calls for an increase of the length of the fuse link. An increase of the length of the fuse link reduces the current-carrying ability thereof, unless the thickness of the fuse link is considerably increased. A considerable increase of the thickness of the fuse link results in a considerable increase of the cross-section of the necks or points of reduced cross-sectional area. A considerable increase of the cross-section of the necks or points of reduced cross-sectional area of a fuse link results in a drastic reduction of the speed of respone and a drastic increase of the peaks of the let-through currents. This defeats the very purpose for which current-limiting fuses of the aforementioned character, i. e. current-limiting fuses having fuse links with point-heat-source-necks, are being designed and applied.

It is the afcrestated dilemma in regard to currentcarrying ability and speed of response which made it difficult, if not impossible, to provide current-limiting fuses having ribbon-type fuse links defining point-heatsource-necks for applications calling for a relatively high current-carrying capacity and involving relatively onerous interrupting conditions.

It is, therefore, still another object of the invention to provide current-limiting fuses, or current-limiting fusible devices, not subject to the aforestated dilemma in regard to current-carrying ability and speed of response.

The foregoing and other general and special objects of the invention and advantages thereof will appear more clearly from the ensuing particular description of the invention, as illustrated in the accompanying drawings, wherein;

Fig. l is substantially a longitudinal section of a currentalimiting protective device embodying the invention,

Fig. 2 is a section along 22 of Fig. 1;

Fig. 3 is substantially a longitudinal section of another currentalimiting protective device embodying the invention;

Fig. 4- is an elevation of the same device as shown in Fig. 3 in another position;

Fig. 5 is substantially a longitudinal section of another structure embodying the invention;

Fig. 6 is a section along 66 of Fig. 7 showing another embodiment of the invention;

Fig. 7 is a section along 77 of Fig. 6; and I Figs. 8a-8c are diagrams of test arrangements tested in evolving the invention.

Referring now to Figs. 1 and 2, reference numeral 1 have been applied to indicate two tubular casings of insulating material which are substantially identical and are arranged in coaxial relation. Each casing 1 is closed by a pair of terminal elements or terminal plugs 2 which are preferably made of solid copper. Reference numeral 3 indicates spacing plates of insulating material to maintain the proper spacing between plugs 2 during assembly of the fuse structure. Steel pins 4 provide strong mechanical ties between casings 1 and plugs 2. Each casing 1 houses a ribbon-type fuse link 5 made of silver foil. Fuse links 5 are provided with V-shaped incisions or notches 5a defining small necks 6. Each neck 6 is sufliciently narrow and sufficiently short to approximate a point-heat-source when links 5 carry current. Links 5 are held in position by grooves 7 provided in the juxtaposed surfaces of plugs 2. Each groove 7 is preferably filled with a soft solder to minimize the resistance between links and plugs 2. Each of the two axially outer plugs is provided with a blade contact 8 for insertion of the dual fuse structure into an appropriate fuse holder or the like. The inside of each casing 1 is filled with a pulverulent arc-quenching filler 9, such as chemically reasonably pure quartz-sand, i. e. quartz sand from which metallic impurities have been carefully removed. The axially inner plugs 2 are conductively connected by a plurality of bars or plates which are preferably made of copper. The ends of plateslt are inserted into grooves 10a provided in the juxtaposed surfaces of the axially inner plugs 2.

The structure shown in Figs. 1 and 2 is supposed to be mounted in such a way as to permit convection current to pass through the gaps formed between contiguous plates 10. These convection currents have been indicated in Fig. 2 by arrows R. The axially inner plugs 13 and the plates or bars 10 form a heat dissipating structure dissipating the heat generated at the point-heat-source necks 6 of fuse links 5. The heat dissipating capacity of the axially inner plugs 2 and the cooling-fin-forming plates 10 is equal to or exceeds the heat dissipating capacity of the axially outer plugs 2 and of the blade contacts 8. The number of plates 9 and the size thereof required to comply with this condition can readily be empirically determined by a few relatively simple tests. The axially inner structure 2, 10 ought to be able to dissipate about twice as much heat as each axially outer plug 2 and blade contact 8 secured to it. The heat dissipated by a fuse holder or the like into which connector blade 3 may be inserted is to be accounted as heat dissipated by the connector blade.

In Fig. 1 the vectors V have been applied to indicate the axially outer heat flow away from necks 6 and vectors V have been applied to indicate the axially inner heat flow away from necks 6 toward the cooling fins 10. V is substantially equal to V The curve T refers to the temperature distribution along the composite currentlimiting fuse structure made up of two substantially iden tical fuses. When the structure is carrying current the temperature distribution along each of the two coupled current-limiting fuse units is substantially the same as if each were applied singly, and the temperature in the median plane of the composite structure is scarcely higher than at the axially outer blades 8. There are steep temperature peaks at the points where the two necks 6 are located. Since the temperature distribution along the composite structure is substantially the. same as if each fuse were applied singly and caused to carry the same current as the composite structure, the timecurrent characteristic of the composite structure is virtually the same as that of each of its components. Due to slight tolerances between the two necks 6 and due to slight differences between the two axially outer vectors V of heat flow the temperature of one of the two necks may differ slightly from the temperature of the other. Such slight differences are insignificant as long as both necks 6 blow on occurrence of relatively large fault currents in such a rapid sequence that the neck to fuse last fuses during the time of initial current decay at the neck to fuse first.

As mentioned before the structure shown in Figs. 1 and 2 must be installed or mounted in such a way as to permit free convection between the cooling plates 10 or, in other words, the structure of Figs. 1 and 2 must be mounted horizontally. The structure shown in Figs. 3 and 4 is not subject to this limitation, i. e. it can be mounted either horizontally or vertically, as desired. The individual fuse units shown in Figs. 3 and 4 are identical to those shown in Figs. 1 and 2 and described in connection therewith, and therefore need not be described here. The axially inner plugs 2 are conductively interconnected by one ormore bars 11' which-are preferably 4, made of copper. A group of cooling fins 12 is mounted on, and supported by, bar or bars 11. This arrangement results in establishing sufficiently similar cooling conditions whether the composite structure is mounted horizontally, as shown in Fig. 3, or vertically, as shown in Fig. 4.

Fig. 5 shows a composite structure which can be applied in any desired position. The structure of Fig. 5 comprises three individual fuse units F of which each is substantially identical with one of the individual fuse units shown in Figs. 1 and 2. As shown in Fig. 5, the structure comprises a tubular casing 13 having the same inside diameter as the outside diameter of the three casings 1. The three current-limiting fuse units F arranged in casing 13 are serially connected by pairs of spaced bars 11 and the spaces between the three currentlimiting fuse units F are filled with a pulverulent filler 14 having a high heat conductivity as, for instance, quartz sand the heat conductivity of which is considerably higher than that of air. The heat generated in the three current-limiting fuse units is conducted by bars 11' to the bodies of pulverulent filler 14, and further transmitted to the outer casing 13, from where it is retransmitted to the ambient.

In the modification of the invention shown in Figs. 6 and 7 a substantially cylindrical copper block is provided with a portion 24) of relatively large diameter and two portions 21 of relatively small diameter. Tubular insulating casings 22 mounted on portions 21 are closed by copper plugs 23 on the sides thereof remote from block portions 20, 21. Steel pins 24 secure casings 22 to block portion 21 and to copper plugs 23. Copper block 20, 21 is provided with an axial passage 25 for a ribbon-type fuse link 26 having two necks 27 adapted to form pointheat-sources. Fuse link 26 is threaded through passage 25 and the axially outer ends thereof are conductively connected to copper blocks 23. One or more wedges 23 driven into passage 25 brace the center portion of link 26 and transfer heat from the center portion of link 26 to the copper block 20, 21. Copper block 20, 21 is provided with a plurality of cooling fins 2% for dissipating the heat flow represented by vectors V The vectors V representing the heat flow from necks 27 toward the axially outer plugs 23 and blade contacts 30 are of substantially the same magnitude as vectors V representing the axially inner heat flow.

Referring now to Fig. 8a, numeral 30 has been applied to generally indicate a current-limiting fuse having substantially the same internal structure as the currentlimiting fuse units shown in the previous figures, and fully described in connection therewith. The currentlimiting fuse has a predetermined current-rating, say 150 amps. When the current-limiting fuse is singly inserted into an electric circuit diagrammatically indicated by the lines 31, and carrying its rated current at a predetermined ambient, say deg. F, the current-limiting fuse will be at a predetermined temperature level. This means that the neck on the fuse link will have a predetermined temperature, the terminal elements will have a predetermined temperature, and the outer surface of the casing at a point midway between the ends thereof will have a predetermined temperature. For the purpose of this analysis it does not matter which of these predetermined temperatures is being considered, as long as when comparing temperatures of current-limiting fuses in different arrangements or structures, the temperatures which are being compared are taken at the same point of the respective arrangements or structures.

Fig. 8b shows a pair of current-limiting fuses 30 which are virtually identical. Both fuses have the same predetermined current rating, and the same predetermined operating temperature when singly inserted into an electric circuit and carrying the rated current at a predetermined ambient. As shown in Fig. 8b, both fuses 30 a are connected in series into an electric circuit 31. Their series connection is effected by a connecting bar 32 having a given length 1. The proportions of bar 32 are entirely arbitrary. Under such circumstances measurement of the various spot-temperatures on fuse units 30 reveals that these spot-temperatures are materially different from the case illustrated in Fig. 8a, though the current-limiting fuse structures 30 as well as the currents which are being carried by them are identical in both instances.

Fig. 8c refers to an arrangement comprising the current-limiting fuses 30 which are identical to the fuses 30 considered in connection with Figs. 8a and 8b, and exhibit the identical properties as the fuses considered in connection with Figs. 8a and 8b when singly used in an electric circuit. According to Fig. 8c the length of the copper connector for serially connecting the two fuses 39 has been increased to L, and the single connector bar 32 shown in Fig. 8b has been substituted by a pair of spaced connector bars 33. This greatly increases the heat dissipating ability of the interfuse cooling structure. As a result of the critical increase of the heat dissipating ability of the interfuse cooling structure the currentlimiting fuses 3t? arebeing maintained at all points thereof at substantially the same temperature levels which they assume when being singly rather than serially inserted into an electric circuit. It will be understood that the interfuse cooling structures shown in Figs. 1 to 7 ought to be adapted to have as high a heat dissipating ability, or as large a heat dissipating area, as the interfuse cooling structure 33 of Fig. 80.

Referring to Figs. 1, 2 and 8c, it will be apparent that the mass and length of the blade-type fuse interconnecting means exceeds the mass and length of the axially outer blade-type contacts used for inserting the composite current-limiting fuse structure into an electric circuit. To achieve the highest possible cooling action, a plurality of spaced bars, or plates, should be used for serially interconnecting the constituent current-limiting fuse unit of the composite current-limiting device.

The structure of Fig. is particularly desirable, inasmuch as the largest diameter thereof hardly exceeds the external diameter of the constituent current-limiting fuse units. It will be apparent that in the embodiment of the invention shown in Fig. 5 the casings of the individual current-limiting fuse units define cylindrical interfuse spaces, and that the heat dissipating interfuse quartz filler is strictly confined to these cylindrical interfuse spaces. The structure of Fig. 5 could be modified, if desired, by decreasing or increasing the number of serially connected integrated current-limiting fuse units. The same applies in regard to the other embodiments of the invention shown.

Having disclosed several preferred embodiments of our invention, it is desired that the same be not limited to the particular structures disclosed. It will be obvious to any person skilled in the art that many modifications and changes may be made without departing from the broad spirit and scope of our invention. Therefore it is desired that the invention be interpreted as broadly as possible, and that it be limited only as required by the prior state of the art.

We claim as our invention:

1. A current-limiting device comprising fusible ribbontype fuse link means, a pair of serially related points of reduced cross-sectional area on said fuse link means sufficiently narrow and sufiiciently short to approximate point heat sources when said fuse link means is carrying current, substantially tubular casing means housing said fuse link means, a pair of heat dissipating metal bodies each arranged at one of the axially outer ends of said fuse link means conductively connected to said fuse link means and closing said casing means, and an additional heat dissipating structure conductively connected to said fuse link t3 means arranged at a point situated between said pair of points of reduced cross-sectional area, said additional structure being adapted to dissipate approximately twice as much heat as each of said pair of metal bodies.

2. A current-limiting device as specified in claim 1 wherein said additional heat dissipating structure comprises a pair of casing-closing plugs and spaced metal bars conductively interconnecting said pair of plugs.

3. A current-limiting device comprising a pair of sub stantially identical aligned current-limiting fuses, each of said pair of current-limiting fuses including an insulating casing, a pair of terminal plugs closing both ends of said casing and a ribbon-type fuse link conductively interconnecting said pair of plugs, said fuse link having one single point of reduced cross-sectional area situated between said pair of plugs and said point of reduced cross-sectional area being sufficiently narrow and sutficiently short to approximate a'point-heat-source when said fuse link is carrying current, and a cooling structure arranged between said pair of current-limiting fuses connecting said pair of current-limiting fuses in series, said cooling structure being adapted to establish an axially inner heat flow from said pair of current-limiting fuses of the same order as the axially outer heat flow from said pair of currentlimiting fuses.

4. A current-limiting device comprising ribbon-type fuse link means, a pair of serially related points of reduced cross-sectional area on said fuse link means sufiiciently narrow and sufficiently short to approximate point-heatsources when said fuse link means is carrying current, substantially tubular casing means housing said fuse link means, a pair of heat dissipating metal bodies each arranged at the axially outer ends of said fuse link means conductively connected to said fuse link means and closing said casing means, and an additional heat dissipating structure conductively connected to said fuse link means arranged at a point situated between said pair of points of reduced cross-sectional area, said additional structure including a group of cooling fins arranged at an angle of 45 degrees with respect to the longitudinal axis of said fuse link means.

5. A current-limiting device comprising a pair of substantially identical aligned current-limiting fuses, each of said pair of current-limiting fuses including an insulating casing, 21 pair of terminal plugs closing both ends of said casing and a ribbon-type fuse link conductively interconnecting said pair of plugs, said fuse link having one single point of reduced cross-sectional area situated between said pair of plugs, said point of reduced cross-sectional area being sufficiently narrow and sufficiently short to approximately a point-heat-source when each of said pair of current-limiting fuses is carrying current, and a cooling structure arranged between said pair of currentlimiting fuses, said cooling structure comprising bar means connecting said current-limiting fuses in series and a group of cooling fins mounted on said bar means enclosing with the axis of said bar means angles of approximately 45 degrees, and said cooling structure being adapted to establish an axially inner heat flow from said pair of current limiting fuses of the same order as the axially outer heat flow from said pair of current-limiting fuses.

6. A currentlimiting device comprising a plurality of ribbon-type fuse link means, a point of reduced crosssectional area on each of said plurality of fuse link means sufiiciently narrow and sufficiently short to approximate a point-heat-source when each of said plurality of fuse link means is carrying current, a plurality of substantially tubular casing means of insulating material each housing one of said plurality of fuse link means, a plurality of pairs of heat dissipating metal bodies, each of said plurality of pairs of metal bodies being arranged at the axially outer ends of one of said plurality of fuse link means conductively connected to said one of said plurality of fuse link means and closing one of said plurality of casing means, an additional casing having an internal diameter substantially equal to the outer diameter of each of said plurality of casing means housing said plurality of easing means,'conductor means arranged in said additional casing for serially connecting said plurality of fuse link means, and a pulverulent filler having a thermal conductivity higher than air arranged outside of said casing means inside of said additional casing.

7. A current-limiting device comprising a pair of sub stantially identicalaligned current-limiting fuses each including a substantially tubular insulating casing, a pair of terminal plugs closing both ends of said casing and a ribbon-type fuse link conductively interconnecting said pair of plugs, said fuse link having one single point of reduced cross-sectional area situated between said pair of plugs, said point of reduced cross-sectional area being sufficiently narrow and suificiently short to approximate a point-heat-source when said fuse link is carrying current, an additional substantially tubular casing housing said pair of current-limiting fuses, and a cooling structure inside said additional casing arranged between said pair of current-limiting fuses, said cooling structure comprising bar means serially connecting said pair of current-limiting fuses, and said cooling structure further comprising a quartz sand filler arranged outside of said casing of each of said pair of current-limiting fuses and inside of said additional casing and in intimate contact with said bar means.

8. A current-limiting device comprising a first currentlimiting fuse; a second substantially identical currentlimiting fuse; said first current-limiting fuse and said second current-limiting fuse each comprising a tubular casing, a pair of terminal elements, and a ribbon-type fuse link conductively interconnecting said pair of terminal elements, said fuse link defining one single neck suificiently narrow and sufiiciently short to approximate a point-heat-source when said fuse link i carrying current; said casing of said first current-limiting fuse being arranged coaxially with respect to said casing of said second current-limiting fuse; a first pair of blade-type connecting means on and projecting axially outwardly from said first current-limiting fuse and said second current-limiting fuse and adapted for connecting said first current-limiting fuse and said second current-limiting fuse into an electric circuit; a second pair of blade-type connecting means arranged in the space situated between said first current-limiting fuse and said second currentlimiting fuse and adapted to serially interconnect said first current-limiting fuse and said second current-limiting fuse; the mass and the length of said second pair of bladetype connecting means exceeding the mass and the length of said first pair of blade-type connecting means, and said second pair of blade-type connecting means being arranged parallel to and spaced from each other.

9. A current-limiting device comprising a first currentlimiting fuse having a predetermined current-rating and a predetermined operating temperature when singly inserted into an electric circuit and carrying the rated current at a predetermined ambient, a second current-limiting fuse substantially identical with said first current-limiting fuse: and having said predetermined current-rating and operating at said predetermined temperature when singly inserted into an electric circuit and carrying said rated current at: said predetermined ambient, said first current-limiting fuseand said second current-limiting fuse each comprising a ribbon-type fuse link having one single point of reduced cross-sectional area sufiiciently short and sufificiently narrow to approximate a point-heat-source when carrying. current, bar connector means for serially connecting said first current-limiting fuse and said second current-limiting, fuse, said bar connector means being adapted to have a sufficiently high heat dissipating area to maintain said first current-limiting fuse and said second current-limiting fuse substantially at said predetermined temperature when said first current-limiting fuse and said second currentlimiting fuse are carrying said rated current in series connection.

10. A current-limiting device comprising a first currentlimiting fuse having a predetermined current-rating and a predetermined operating temperature when singly inserted into an electric circuit and carrying the rated current at a predetermined ambient, a second currentlimiting fuse substantially identical with said first currentlimiting fuse and having said predetermined current-rating and operating at said predetermined temperature when singly inserted into an electric circuit and carrying said rated current at said predetermined ambient, said first current-limiting, fuse and said second current-limiting fuse each comprising a substantially tubular casing, a pair of terminal elements closing both ends of said casing and a ribbon-type fuse link having one single point of reduced cross-sectional area sufficiently short and sufii ciently narrow to approximate a point-heat-source when carrying current, said casing of said first current-limiting fuse and said casing of said second current-limiting fuse being arranged in coaxial relation and jointly defining a cylindrical space, substantially straight bar connector means for serially connecting said first current-limiting fuse and said second current-limiting fuse, amedium having a higher heat conductivity than air in physical engagement with and surrounding said bar connector means, and said medium being confined to said cylindrical space defined by and situated between said casing of said first current-limiting fuse and said casing of said second current-limiting fuse.

References Cited in the file of this patent UNITED STATES PATENTS 2,501,963 Pittman Mar. 28, 1950 2,577,531 Laing Dec. 4, 1951 2,665,348 Kozacka Jan. 5, 1954 2,734,112 Kozacka Feb. 7, 1956 FOREIGN PATENTS 337,386 Great Britain Oct. 29, 1930 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nos 2,871,314 January 27, 1959 Kenneth W, Swain et a1.

It is herebfi certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 51, for "approximately" read approximate Signed and sealed this 19th day of May 1959.

SEAL) Attest:

KARL Ha AXLINE Attesting Oflicer ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,871,314 January 27,, 1959 Kenneth W. Swain et al.

It is hex-e11 certified that error appears in the-printed specification of the above "numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 51, for "approximately" read approximate Signed and sealed this 19th day or May 1959.

(SEAL) Attest:

KARL Ho AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

